Hardened seats coming loose

[Daverz]

I have the heads off of my’49 and the seats are need of replacement. I have also read the multiple posts of these dropping out yet I see that Carl Olsen’s shop and V-twin’s shop both install seats. How do they get by with it- or do they?

[Chris Haynes]

Replacing Panhead seats is easy. Trying to get them to stay in place is something else altogether.

[BigLakeBob]

There is no need for hardened seats, a myth started when lead was removed from fuel. Aluminum bronze works very well and will expand as the heads expands. Panheads have a bad reputation for dropping seats, but in my opinion its only because the wrong material is being used, a hardened seat will not grow in heat and will probably come loose.

[T. Cotten]

Daverz!

If you had some OEM replacement seats, they can be safely installed within the original cast-in seats.

Unfortunately, all modern offerings, whether hardened steels or bronze alloys are spec'd to be installed at a drastic interference within the aluminum casting, with the original seat cut away.
Plumber's-port heads do not have the structural support, and can crack disastrously upon installation.

A reason for this extreme interference for steels is that the seat will shrink permanently if it reaches a critical temperature, due to advanced timing or a vacuum leak. Apparently there is some similarity with the bronzes, but I have never cooked one to measure its thermal expansion.
I did find that a six thou interference on a steel seat drops to less than one at running temperatures, and that is without any shrinkage from a temperature spike.

Not only must the seat be held by the casting, it must remain tight enough to transfer its heat to the head.
Later Pan castings have much more "meat" to work with.

Have you investigated oversize valves?

....Cotten

[Daverz]

Thanks Cotton, That is the most extensive explanation I have heard, good information and point well taken. Do you have a source for oversize valves?

[T. Cotten]

Daverz!

I haven't 'sourced' anything like that for years, as I have been distracted.

There were OEM oversized valves, and various aftermarket productions over the decades.
A talented headmeister would probably trim down large ones so some of your seat's life was saved.

Finding "talent" is why most of us either do it for ourselves and take the blame, or it don't get done.

....Cotten

[duffeycycles]

on my head that had the seats replaced,the heads I came up with at the time were all cracked.My machinist cut the entire seat area out of the head so as not to have weld contamination.It was put in a kiln welding device at a major manufacturing plant.Then he welded thru a hole in the kiln & slow cooled[govt job 3rd shift!]The head was machined for .0075" interference..Then heated in a kiln & then pressed in.I was scared to run it at first...20 years later,most pulling a rig I think it worked fine!

[shovelheadstan]

I done some shopping around on ebay. Found 1/8 inch oversise valves. V Twin repops. lowered the valve stem height around 100 thousandths.

[Carl Olsen]

The trick to replacing any valve seat is using a material that has a matched coeficient of heat expansion to the head it is going in, a round concentric hole with the proper press fit.
Carl

[T. Cotten]

The trick to replacing any valve seat is using a material that has a matched coeficient of heat expansion to the head it is going in, a round concentric hole with the proper press fit.
Carl

That would be the Great Trick indeed, Carl!

Do you know of a seat replacement or material that has a thermal expansion coefficient anywhere near cast aluminum?

I left that zodiac long ago in fear and frustration, so thanks in advance!

....Cotten

[BigLakeBob]

That would be the Great Trick indeed, Carl!

Do you know of a seat replacement or material that has a thermal expansion coefficient anywhere near cast aluminum?

I left that zodiac long ago in fear and frustration, so thanks in advance!

....Cotten

Carl is 100% correct, no trick, read my post #3 for material.

[BigLakeBob]

Bob 3.jpg Just did this plumber style last week, no negative issues.

[moldthread]

The seat material to use in aluminum heads is ALUMINUM BRONZE.
Correct alloys are either C-954 (generic classification) or AMPCO 18 (trade name).
Expansion rate for both is .000009"/"/deg. F.
Expansion for 356 cast aluminum is .000012"/"/deg. F.
On a 2" dia. seat I use between .004-.006 interference fit.
Shrink insert in dry ice and heat head to 250 deg. F. and work fast.

[T. Cotten]

Please explain to me, Folks:

If the thermal expansion coefficient of "aluminum bronze" is similar to cast aluminum, then why is the dangerously high interference fit, same as hardened seats, necessary?

(Please note once again, OEM bronze seats were intended to be pressed into the existing captured bronze seats at a normal couple of thou of interference, not the bare aluminum.)

....Cotten

[moldthread]

The amount of press fit is dependent on the diameter.
The larger diameter the larger the press
Think about the press fit on a valve guide for Harley flat heads.
Diameter is between .5625 & .5937.
Press fit is .0025 / .0015
Now think about a valve seat that is about 2.000 dia. or larger (4 times the diameter of the guide).